Traditionally, the flow method and the reflow method have been used as the soldering methods most suitable for industrial mass production. For example, when the flow method is used for mounting components onto a printed circuit board (hereafter referred to as "substrate"), the substrate on which components have been placed is coated with flux, and then soldering is achieved by passing said substrate through a molten solder bath. In the reflow soldering method, a mixture of flux and soldering paste is printed onto the part of the substrate on which components are to be soldered. Then, after the components are placed on the substrate, soldering is achieved by passing said substrate through a heating oven called a "reflow oven." However, it is difficult to stably produce a uniform coating of flux on the bonding surface. For this reason, this coating step has become a major obstacle to achieving automation, as substrate patterns have become finer and assembly density has increased. Furthermore, in both methods, the residual flux on the substrate is corrosive, and thus, it is necessary to rinse the substrate after soldering. Additionally, when soldering a TAB substrate to a liquid crystal panel, the risk exists that the flux may splash and contaminate the polarizing film. Consequently, it is necessary to attach a protective film to the polarizing film beforehand, and to remove it after soldering is finished. This requirement increases the number of process steps, time, and labor, resulting in increased manufacturing costs.
Known methods of removing organic substances, such as flux, include the wet rinse method which uses an organic solvent, and the dry rinse method which removes organic substances by causing a chemical reaction in them through irradiation with ozone, or ultraviolet light, etc. However, the wet rinse method requires an additional rinsing process for removing the rinsing agent after the organic substances have been rinsed off, a process for drying the substrate, and a stationary facility for performing these processes. These additional requirements necessitate a massive amount of time and labor and result in increased manufacturing costs. On the other hand, thorough rinsing cannot be expected in the dry rinse method since it is difficult to completely remove organic substances possessing especially large molar weight.
To avoid these problems, no-rinse fluxes containing very little or no active species, such as chlorine, have been in use recently. However, because these fluxes possess poorer wettability than conventional fluxes, they have raised concerns that the resulting solder may be incomplete or may possess insufficient bonding strength.
Furthermore, when components must be soldered on both sides of a substrate, the heat treatment applied during the process up to that point causes the formation of an oxide film (CuO) on the surface of copper pads and electrodes, along with the loss of even the initial level of wettability; and in some cases even leads to increased contact resistance during soldering.
Unexamined Japanese patent application No. H03-174972 discusses a method which obtains excellent adhesion of a soldering material to a substrate. In this method, the substrate surface is coated with the soldering material, after using an argon gas atmosphere in vacuum to generate an electrical discharge, and this discharge is used to remove impurities from the surface of the substrate and to improve its wettability. However, this method requires a vacuum pump and a vacuum chamber for obtaining the vacuum atmosphere, making the overall equipment large and complicated. Furthermore, argon, the gas used, does improve the wettability between the soldering paste solvent and the substrate, thus improving the printability of the paste solder. However, it does not improve the wettability of the solder substrate since it does not remove the oxide film. Therefore, it is necessary to coat the soldering surface with a flux in order to remove the oxide film before soldering, and consequently, this flux must be rinsed off after soldering.
Furthermore, unexamined Japanese patent application No. S3-127965 discusses a device that is equipped with a second switch for generating arc discharge with the component to be soldered, and for quickly heating said component. This device makes it easy to solder said component, even if its heat capacity is large.
However, with this device, the electrical discharge is used to generate the heat required for soldering, and cannot be used to remove the oxide film. Furthermore, since the gas used is inert, removal of the oxide film is impossible in theory. This method also requires that the soldering surface be coated with a flux in order to remove the oxide film, which must then be rinsed off after soldering.
The invention solves these above mentioned problems associated with conventional technologies, and its objective is to provide a method and a device for the soldering technology used for bonding two components, that can easily and thoroughly improve the wettability of the surfaces to be bonded.
The second objective of the invention is to provide a method and a device for soldering using a low corrosive, no-rinse flux, or without the use of a flux.
It is another objective of the invention, in cases involving the bonding of electronic components to a substrate by soldering, to provide a method and a device that improve the wettability of the substrate without damaging it, and that make it possible to always obtain good soldering results, even for bonding involving fine wiring patterns or minute areas.